Direct conversion transmitters are widely used in the field of wireless communication. In a direct conversion transmitter, baseband in-phase (I) and quadrature-phase (Q) signals are up-converted into radio frequency (RF) signals with a carrier provided by a local oscillator. Then the up-converted I signal and Q signal are then summed, amplified as an output signal and the output signal is transmitted via an antenna.
Carrier leakage caused by local oscillator error, baseband mismatch etc. is a concerned issue for the direct convention transmitter. To eliminate or reduce carrier leakage, a common solution is to detect an output of the direct conversion transmitter when no I and Q signals are input thereto, so as to obtain the leakage, and generate a compensation signal based on the detected leakage, led to reduce or even eliminate the leakage. The detection and compensation procedures can be repeated until the leakage is minimized.
For some applications, such as WCDMA, it is necessary to provide an RF variable gain amplifier (VGA) on a signal output path of the direct conversion transmitter to adjustably generate an output signal with a variable gain under a transmission condition. Further, under a calibration condition, before detecting the output signal for leakage upon no I and Q input signals fed into the transmitter, it is optimal to down-convert the leakage at radio frequency (RF) into a lowered frequency, such as an intermediate frequency (IF) or a baseband (BB) frequency for the convenience of process in the subsequent steps.